Method and apparatus to determine carbon potential in the atmosphere of treatment furnaces

ABSTRACT

AN ELECTRICAL SIGNAL REPRESENTATIVE OF THE LOGARITHM OF THE PARTIAL PRESSURE OF THE COMPONENT IN THE FURNACE IS DERIVED AND TO THIS LOGARITHMIC SIGNAL AN ELECTRICAL SIGNAL DIRECTLY RESPONSIVE TO TEMPERATURE IS ADDED TO PROVIDE FOR A TGEMPERATURE CORRECTION FACTOR, OTHER SIGNALS REPRESENTATIVE OF OTHER CORRECTION FACTORS, FOR EXAMPLE TO COMPENSATE FOR DRIFT OF MEASURING INSTRUMENTS, MAY BE ADDED, THE COMPOSITE SIGNAL IS THEN INDICATIVE OF THE CARBON POTENTIAL OF THE ATMOSPHERE WITHIN THE FURNACE.

J. WUNNING 3,605 484 METHOD AND APPARATUS TO DETERMINE CARBON POTENTIALIN THE Sept. 20, 1971 ATMOSPHERE OF TREATMENT FURNACES 2 Sheets-Sheet 1Filed Q61.- 5, 1968 (pventor Acfi/M WOWIV/NG by 7i? p 1971 J. WUNNING3,605,484

METHOD AND APPARATUS TO DETERMINE CARBON POTENTIAL IN THE ATMOSPHERE 0FTREATMENT FURNACES Filed Oct. 3. 1968 2 Sheets-Sheet 2 I PROPANE ICOMPARATOR. MTENTIOHETER M THERMOCOUPLE I PARTIAL I 6 g c I Pgg ggg: 44a l 40 FURNACE E a AL flcIIr ATMOS H E I 25 I ER mg? 1 I I I I I 1 N 1s29 n so 18 51 -52 CONTROL \V I I I4 6 FURNACE ATMOSPHERE TEST SAMPLE F1-240 140 EaAND COMPARISON J COMPOSITION ADJUSD A! --wv wv *w M, 2?. 2 I

United States Patent M 3,605,484 METHOD AND APPARATUS TO DETERMINECARBON POTENTIAL IN THE ATMOSPHERE OF TREATMENT FURNACES JoachimWiinning, Bergstrasse, 7251 Warmbronn, Germany Continuation-impart ofabandoned application Ser. No. 631,240, Apr. 17, 1967. This applicationOct. 3, 1968, Ser. No. 784,496 Claims priority, application Germany,Apr. 21, 1966, W 41,390; Oct. 5, 1967,P 16 73 328.1

Int. Cl. G01n 7/00 U.S. CI. 73-23 14 Claims ABSTRACT OF THE DISCLOSUREAn electrical signal representative of the logarithm of the partialpressure of the component in the furnace is derived and to thislogarithmic signal an electrical signal directly responsive totemperature is added to provide for a temperature correction factor;other signals representative of other correction factors, for example tocompensate for drift of measuring instruments, may be added; thecomposite signal is then indicative of the carbon potential of theatmosphere within the furnace.

PRIOR U.S. APPLICATION The present application is a continuation-in-partof U.S. Ser. No. 631,240 filed Apr. 17, 1967, now abandoned.

The present invention relates to a method, and to apparatus to determinethe carbon potential of the atmosphere in furnaces for the treatment ofmetals, particularly ferrous metals, and more particularly relates to amethod and apparatus to determine the concentration of a component ofthe gas Within the atmosphere, for example carbon dioxide (CO or watervapor or steam.

Furnaces for treatment of metals, particularly ferrous metals, which maybe classified as annealing. furnaces although the work pieces may beheated to incandescence, contain an atmosphere which is in essentialequilibrium. The carbon potential within these furnaces must bedetermined, and it is of particular importance to take intoconsideration ambient conditions existing within the furnace, such asthe temperature therein, the composition of the atmosphere Within thefurnace, the pressure thereof, as well as the composition of the workpiece to be treated.

German Pat. 1,071,378 (corresponding to U.S. Pat. No. 2,935,866)contains a discussion related to measuring and controlling the addition,or removal of carbon from carbon soluble work pieces, such as iron, oriron alloys, in annealing furnaces. As set forth in this patent, it isnecessary when controlling the carbon content within the work piece toprovide for a means to measure increase or decrease of carbon contentaccurately. Carbon is absorbed. or removed from the work piece, until anequilibrium condition exists between the carbon concentration within thework piece and the mixture of the atmosphere surrounding the work piecewhich includes carbon atoms itself. The carbon content which a sampleconsisting of pure iron can absorb, at a given temperature, when inequilibrium with an atmosphere within the oven in generally referred toas the carbon potential or the C-potential of the atmosphere within thefurnace. Various arrangements and systems to measure this carbonpotential are known, in order to adjust the atmosphere within thefurnace to a specific carbon potential; reference may be made to HTM,Part A, Volume 16 (1961), Issue 1, page 7. Two methods of measuring, thedirect and the indirect methods, have to be distinguished.

Direct measuring of the carbon potential is done placing 3,605,484.Patented Sept. 20, 1971 a sample in the furnace, bringing the atmospherewithin the furnace into equilibrium therewith, and then determining thecarbon content of the sample either, after cooling or quenching byanalysis, or by determination of electrical conductivity within thefurnace itself (see, for example, the aforementioned German 1,071,378, U.8. Pat. No. 2,935,866). In the indirect method, the composition of thegaseous atmosphere within the furnace itself is analyzed and the carbonpotential is then calculated from known equilibrium conditions of thegaseous components within the furnace atmosphere.

The direct method of determining the carbon potential has the advantageof accuracy and simplicity; yet, it has substantial disadvantages whichprevent its use in many instances. For example, the method of analysis,for example, by analyzing foils, cannot be done continuously andautomatically; when introducing a separate test wire within the carbonatmosphere in order to make an electrical conductivity determination,the presence of such test wire may be objectionable and further, a highcarbon potential which reaches the region of iron carbides, cannot beaccurately measured. Such a high carbon potential, which is desired whenforcing increased carbon content has a deleterious effect on the sensingwire and may cause suflicient damage to prevent good measurement.

The indirect method of measuring carbon potential is generally used inindustry. It is known to utilize the components of the gases within thefurnace atmosphere as test samples. As can be shown, it is practicallyonly useful to use the CO CH or the H O components as a representativequantity to determine the carbon potential of the atmosphere within thefurnace. The concentration of the particular component is determined.The CH component is, due to the slow change of equilibrium condition,not used much in practice, so that as measuring samples the CO and the H0 components are usually the only ones which can effectively beutilized. Today, the 00 or H O concentration is almost exclusivelydetermined by means of measuring the dew point, for which a lithiumchloride measuring unit is used; alternatively, infrared analysis may beemployed. For literature references regarding the analysis of theparticular component itself, reference may be had toChemie-Ingenieur-Technik" No. 6, 33 volume (1961), pp. 426-430. When thedew point, in C., or the partial pressure of the carbon dioxide isdetermined, the carbon potential is then obtained from families ofcurves, which relate carbon potential to the measured quantities forvarious temperatures of the furnace, which are the most importantparameters to be determined. These diagrams assume as a precondition,

gas within the atmosphere of the furnace itself, the absolute pressure,the particular alloy composition of the material to be treated, etc. isthe same as that on which the families of curves are based. This,however, is not the case in many instances. Yet, it is difficult toconsider all the other factors, in actually carrying out these measuringprocesses, because the correction curves and diagrams necessary wouldunduly complicate the measuring techniques.

The indirect measuring methods, used today, require relativelycomplicated instrumentation and still have the disadvantage that theaccuracy of measurement, as obtained in plants themselves, is frequentlynot suflicient to satisfy requirements. Additionally, the use of curvesand diagrams to determine the carbon potential from the measuring valuesobtained by sensing elements is timei which is a substantial factor inthe accuracy of the final carbon potential determination. Thistemperature determination is frequently insufficiently accurate becausethe temperature within the furnace is determined, in acutal practice, atonly one particular point in the furnace which is usually remote fromthe point of removal of the gas, thus introducing errors which may reach10 C. or more. The present invention departs from the prior method andprovides for temperature compensation, the temperature being determinedfrom the gases in the oven themselves. For accurate determination, thetemperature sensing elements must also be accurate. Their calibrationmust be accurately known, and they should not be subject to drift;Further, thermal dynamic equilibrium within the furnace must obtain, sothat temperature measurements are made under equilibrium conditions andnot during deviations from an average value.

It is an object of the present invention to improve the accuracy ofmeasuring carbon potential under continuous operating conditions of thefurnace without the use tabulations involving curves and diagrams.

It is a further object of the present invention to provide a measuringmethod, and apparatus, in which errors arising due to lack ofcompensation for miscalibration of the temperature sensing elements, ordue to deviation from thermal equilibrium are compensated for.

SUBJECT MATTER OF THE INVENTION Briefly, in accordance with the presentinvention, gas is removed from a given location within the furnace,which given location is also utilized to measure the temperature withinthe furnace. Thus, the temperature determination and removal of the gas,the composition of which will be determined, is practically co-instant.The gas is analyzed and a value is determined proportional to theconcentra tion of the gas component under analysis; this value is madeapproximately equal to the logarithm of the partial pressure of thegaseous component and this, so transformed measuring value is added to ameasuring value derived from the temperature, to add a temperaturecorrection factor thereto; from this combined added value, the carbonpotential can be directly indicated. Other correction factorsrepresentative for example of composition of the atmosphere within thefurnace, the absolute pressure within the furnace, the particular alloycomposition of the work piece, etc. can readily be added to the measuredvalue.

In accordance with a feature of the invention, a correction factor isobtained representative of drift of the temperature measuring element,or of lack of equilibrium of the carbon potential, by making adetermination from a test sample, from time to time, and compensating,electrically, for any deviations between indicated, and actuallymeasured values.

In accordance with another feature of the invention, an electricalsignal (current, or voltage) is derived representative of theconcentration of the, particular gas, which signal is made proportionalto the logarithm of the partial pressure thereof. A' correction signalderived from a thermocouple is added to the signal representative of gasconcentration. By making the signal representative of the carbon dioxideor vapor component of the atmosphere proportional to the logarithm ofthe partial pressure, correction of the final value, necessary toconsider the influence of other parameters within the furnace can beeasily carried out by electrically adding signals.

Correction for instrumentation drift, or lack of equilibrium is obtainedby adding another electrical correction signal to the signalrepresentative of gas concentration. This signal can be varied, fromtime to time, in accordance with analysis results obtained from testsamples, and comparing these results with the value indicated by theinstrument. The additional signal is then added in (in positive, ornegative direction) in order to have the indication on the "instrumentagree with actual conditions as determined from analysis of the testsample. The carbon possible to utilize a regulator which adjusts thecomposition of the atmosphere within the furnace. One of the advantagesof the method and apparatus according to the present invention consiststherein that the scale of the indicating instrument can be directlyreadable in percent carbon of gamma-iron so that a direct reading isobtained.

It has been found under actual operating conditions that an accuracy ofi0;05% carbon can be obtained.

According to the invention, carbon potential can be determined withoutthe use of complicated diagrams, or computer apparatus replacing.diagrams. It has been found, suprisingly, that the heat of mixing ofgamma iron is equal to zero. The method and apparatus of the presentinvention provides for technical utilization of this discovery.

The mathematical derivation will be given. In this derivation thefollowing definitions are used: p, (in atmospheres) is equal to partialpressure of the gas component i in the furance c (in atomic fractions)is equal to concentration of carbon in gamma-iron T K.) equals absolutetemperature in the furnace 7;, is a correction coefficient for aspecific alloy composition, of the work piece, and will he usually usedas (in percent C.) defined as Percent 0 (pure iron sample) ,7 0

Percent C (alloy sample) The basis for the functioning of the presentinvention is the Gibbs-Helmholtz equation:

AG: AH-TAS wherein AG is the reaction enthalpy;

AH is the heat of reaction; and AS is the reaction entropy.

From this equation the transition of carbon from the atmosphere withinthe furnace in the gamma-composite crystal of the iron can be derived:

Pco

The water vapor reaction is considered in the following relationship 10gpflzFlog (1:58 o.

where the logarithm is to the base 10. I It will be noted that the aboverelationship consists essentially of factors which are being added, eachone depending on a particular parameter existing within the furnace. Thefactor corrects for activity of the workpiece; How this is done will bedescribed in detail below.

The partial pressures p or P112 respectively, are made proportional toelectrical currents or potentials. Correction is then particularlysimple because it can be entered by manually settable potentiometers.Any one of the correction factors has a potentiometer assigned thereto,the scale of which can be directly readable in the correction factor, sothat the use of additional diagrams is unnecessary. Further, automaticcompensation of at least some of the correction factors can be carriedout automatically very simply, because within a certain range a shift ofthe zero or null point is proportional to the specific quantity to bemeasured, so that the correction factors to be entered can remainconstant and need not be changed. I

The method according to the present invention can be carried out readilyby means of an apparatus which, in accordance with another feature ofthe present invention, consists of a sampling assembly, introduced intothe furnace, which contains a thermo-electric element within aprotective cover and, an immediately adjacently arranged suction tubehaving at least on its interior surface a ceramic coating, from whichgas is removed by suction from the furnace. Concentration of the gas soremoved is determined in an analysis apparatus, which may be any form ofapparatus well known in the art itself. The analysis apparatus is soarranged that it delivers a signal at least substantially proportionalto the logarithm of the partial pressure of the gas component underconsideration, in the analysis apparatus. Electrical signals so obtainedare then added in an adding network to a potential proportional to theoutput of the thermo-electric elements. An indicator will then readdirectly in percent of carbon potential, automatically corrected at alltimes for temperature variations. The temperature, or the ham of whichthe gas analysis is done, is thus accurately determined, becausetemperature differences which might be caused by determining temperatureat a point different from the gas removal itself is avoided. Since thedetermination of carbon potential is highly dependent on temperature, asindicated in the mathematical analysis above, an accurate determinationof temperature 18 1mportant in order to obtain accurate measuringresults.

Introducing a sampling assembly within the furnace enables constructionof a simple unitary test transducer, which does not require substantialmodifications of, or within the furnace itself.

FIG. 1 is a partially vertically sectional view of a test assembly incombination with the apparatus, shown partly in schematic form;

FIG. 2 is an electrical circuit diagram, in schematic form, indicatingmeasuring of water vapor concentration; and

FIG. 3 is a modified circuit diagram, particularly suited for measuringcarbon dioxide concentration within the atmosphere in the furnace, inschematic form.

Referring now to the drawings and particularly to FIG. 1:

The furnace, not forming part of the invention, is not illustrated; itswall 2 is formed with an opening into which a sampling and test assemblygenerally shown at 1 can be inserted. The test assembly 1 includes a gassuction tube 3, 4, formed of two telescoping tubes; the outer one, tube3, may consist of chromium nickel steel, and the inner one, tube 4, maybe of ceramic material in order to prevent changes'in the gas beingremoved which might arise if metallic surfaces, at the temperature ofthe oven, have catalytic effects thereon. The gas inlet is shown at 5.Immediately adjacent the gas inlet 5, and within the common samplingassembly 1 and secure to a common sampling assembly head 11, is adouble-walled protective tube 7 containing a thermo element, the activepoint 6 of which is located adjacent the suction inlet 5 of gas tubes 3,4. Since both tubes 3, 4 and the tube 7,

protecting the thermo element, extend within the furnace immediatelyadjacent each other, the temperature at the active point 6 of the thermoelement correspond, in actual operating conditions, exactly to thetemperature of the gas being removed at point 5, so that the temperatureof the gas as actually removed is determined. The sampling assembly head11 contains a third tube 8, extending within the interior of thefurnace, the interior of which communicates with the furnace interior bymeans of perforations 9 formed in the Wall therein. Tube 8 is adapted tohold work samples, shown at 10, which can be introduced into the furnaceto initially adjust, and calibrate the instrument, as will be discussedin detail below. The sampling head 11 is formed with a removable cover20 and each one of the tubes extending within the furnace itself iscovered by a plug 21.

Gas removed from the interior of ceramic tube 4, and entering throughinlet 5 is first led to a gas filter .12, arranged immediately withinthe sampling head 11. It is then taken off by means of an outlet pipe13, and sucked by means of a pump 22 to an analysis apparatus 14. Sincefilter 12 is arranged immediately within the sampling head 11, which isitself heated by contact with the metallic tubes 4-, 7, 8, condensationof gas Within the filter 12 is avoided yet it is unnecessary to addadditional separate heating means for the filter. I I

The analysis apparatus 14 which will be discussed in detail below,supplies an electrical voltage (or current) to operate indicator 15(FIG. 1). This indicator can be directly set to indicate percent ofcarbon potential. It is arranged in a cabinet 23 and contains thecircuits further illustrated in FIGS. 2 or 3. A value representative oftemperature is applied thereto over conductors 140, connected to theactive point 6 of the thermo electric element. Potentiometers 16, 17 and18, are manually settable in accordance with directly indicating scales24 likewise arranged on cabinet 23 in order to compensate for parametersor ambient conditions existing in the oven, such as the particularcomposition of the atmosphere in the oven, pressure within the oven, orcomposition or particular alloy of the work piece within the furnace.

Referring now to FIG. 2, the partial H O pressure is determined by alithium chloride humidity-measuring element and, in this case, formingthe analysis instrument 14. A humidity sensing resistance 14a isincluded in one branch of a bridge network 25, one diagonal connectionof which is fed by a rectifier 26 supplied by potential from one windingof a transformer 50. The structure of the rectifier itself need notfurther be explained and it may be an ordinary kind of a bridgerectifier; it is indicated schematically only in FIG. 2. The otherdiagonal portion of the bridge 25 is connected in series to a potentialwhich is determined by the temperature, as obtained from thermo element6. The conductors from thermo element 6 are connected across a variableresistance 27 which is again fed by potential from a rectifier 28connected to the transformer 50. Thus the sum of the potentials obtainedfrom instrument 14, in essence from the humidity sensitive element 14aand the potential determined by the thermo electric element 6 are added.To this added signal, a group of three further signals are added whichconsist of potentials obtained across the diagonals of three furtherbridge networks 29, 3d, 31. The particular value of each one of thepotentials is determined by potentionmeters 16' 17, 18 contained in onebranch of the bridge network. Each of the bridge networks are suppliedby separate rectifiers 33, 34, 35, connected across the three diagonalsof the bridges 29, 30, 31. The sum of all these potentials, that is thesum from bridge 25, the potential corresponding to the thermo electricelement 6 and from bridges 29, 30, 31 then applied over an amplifier 32to an indicator 15 which is directly readable in percent of carbonpotential. The amplifier 32 can also supply the controller in order tomaintain the indicated percent carbon within specific limits byregulating input of air or other gases to the furnace as will appear inmore detail in connection with FIG. 3.

The circuit according to FIG. 2 is arranged to permit the addition offixed values to the output obtained from analysis instrument 14 and thethermo electric element 6. The analysis instrument 14 is so arrangedthat the output obtained therefrom corresponds to the relationshipindicated in Equation 2 above. That is, an output potential proportionalto log PHZO. Detailed consideration of Equation 2 shows that, when theaverage operating conditions are approximately within a medium range,which is usually the case in actual practice, that the other factorswithin Equation 2 can be so transformed so that constant correctionfactors can be added therein. These correction factors are obtained byproper adjustment of potentionmeters 16, 17 and 18 in accordance Withscales 24; it is of course also possible to continuously vary thesetting of potentiometers 16, 17 and 18 depending upon the output ofother sensing elements or transducers which influence the setting of thepotentiometers in accordance with variation of ambient conditions, suchas for example variation of ambient pressure within the furnace.

The composition of the work piece has to be compensated for; ordinarilyit is customary to define the carbon potential with respect to pureiron. In ordinarily used nonalloy steel, the effects of deviations dueto activity of the material are within the limit of tolerance of theequipment. Yet, the addition of alloying materials even in only littlealloyed steels have an influence which can no longer be ignored. Theactivity of carbon in the gamma-crystal can be sufiicient so that thecarbon content may change up to 20 percent from the desired value, ifthe alloy content is not considered. Alloying elements which formcarbides have an even greater effect.

The influence of alloying elements on the activity of the carbon isgenerally considered in thermodynamics by means of a correctionco-efiicient. There are numerous publications disclosing experimentalresults and considering mathematical relationships determined by theinfluence of various alloying elements (see, for example, Arch. forEisenhuttenwesen, Vol. 35 (1964), pp. 999-1007 Correction for theactivity constants in the above Equations 1 and 2 appears as theadditive constant. When measuring the carbon potential, such correctioncan be done, like the other corrections, by shifting the zero or nullpoint; in practical effect, and as illustrated in FIG. 2, this isreadily accomplished by adding a correction potential.

It would be possible to compensate for the influence of several alloyingelements in any one steel composition separately, that is, to provide aseparate potentiometer for each alloying element. Since, however, themutual influence of the various alloying elements must also beconsidered, it is usually easier in actual practice to determine acorrection factor for any particular type of steel directly and then toadjust a corresponding potentiometer in accordance with a predeterminedscale setting.

As has been noted, in practical effect it is useful to utilize acorrection factor for the carbon potential itself, rather than acoefficient which compensates for the activity within the carboncrystal. In accordance with the present invention, this coefiicient forcorrection for the carbon potential can readily be determined. Referringnow to FIG. 1, the third tube 8 includes a test. sample 10 consisting ofpure, that is, unalloyed iron; further, the test sample consisting ofthe work piece is inserted therein. Both test samples are exposed to thetemperature and atmosphere of the furnace for a sufficient time in orderto obtain equilibrium. Thereafter, the test samples are removed fromtube 8, quenched and analyzed for carbon content. The coelficient to bedetermined then is proportional to the quotient of the carbon content ofthe unalloyed, that is, pure sample (which is the same as that of thecarbon potential for pure iron, of course) and of the alloyed work pieceor sample. Mathematically, the relationship is percent C (pure ironsample) percent 0 (alloy sample) which, of course, corresponds to thedefinition given above.

Since this coefficient is, in part, dependent on carbon concentration,it is recommended to utilize a standard carbon addition, such as forexample 1% C and to make the determination of the coeificient at leastapproximately in the region of such a value.

The scale of the correction potentiometer, which is intended to correctfor the composition of the alloy, may contain not only the coefficientabove-referred to but may further contain indications showing theparticular types of steel to be treated. This greatly simplifies settingby even unskilled personnel.

If a carbon nitrating process is used, then the influence of thenitrogen absorbed by the iron and corresponding to the addition ofammonia can readily be compensated for.

Referring now to FIG. 3; this figure, basically corresponds to thecircuit of FIG. 2. For simplicity, a single bridge network, that isbridge 29, has been illustrated in order to consider the correctionfactors previously discussed. Of course, in FIG. 3, as well as in FIG.2, various serially connected bridge circuits can be used. A furtherbridge network 37 is added to the bridge networks 29, in similar manner.A source of electrical potential 137 is connected across one of thediagonal cross con nections of bridge 37, the other diagonal crossconnection being placed in series with bridge circuit 29 and theconnection to element 32; A variable potentiometer 38 is adjusted inorder to introduce a correction potential, the value of which isdetermined by deviation between indicated value, that is value indicatedby measuring instrument 15 and actual value as determined from analysisof a sample of iron strip, or foil introduced into the furnace.

The additional compensation compensates for aging and drift of thethermal elements providing for temperature measurement, as well as forerrors which arise due to lack of thermo dynamic equilibrium within thefurnace. The accuracy of measurement obtained is thus, practically,limited only by the possible errors in analysis of the iron samplestrip. It has been found in actual practice that adjustment ofpotentiometer 38, to compensate for variations in thermo dynamicequilibrium, and in calibration of the temperature measuring elements isonly necessary once or twice weekly. Changes in calibration of themeasuring instrument, and changes in thermo dynamic equilibrium withinthe furnace occur only slowly.

The anaylsis instrument 14, instead of containing a humidity sensitiveresistance, may be made to be sensitive to the partial pressure of thecarbon dioxide component. The analysis instrument 14 then contains aninfra-red analysis device 240, operating in accordance with infra-redabsorption and delivering a potential which is dependent on the partialpressure of the carbon dioxide component introduced therein. By suitablechoice of the length of the measuring chamber within unit 240, theoutput is made approximately proportional to the logarithm of thepartial pressure of the carbon dioxide, that is, mathematically to log pThe relationship of the output will then be in accordance with Formula 1above.

Although the output should relate only to a particular component withinthe gaseous mixtures introduced thereto, the composition of the gas, inthe aggregate, has a substantial influence on the result of the outputfrom analysis apparatus 240.

A so-called cross-sensitivity exists which in the past 7 (percent 0) Ihas been compensated by special arrangements within the instrumentitself, which introduced complications. For example, the particular gasto be considered can be absorbed specially; or, the undesired gases canbe introduced into a special comparison or test vessel for sampling; oradditional and known gases can be introduced or used for comparison. Inaccordance with the present invention, such undesired cross-sensitivitycan be used directly for correction and calibration of the measuringinstrument. The cross-sensitivity for the partial pressure of carbonmonoxide is suppressed, that is compensated only insofar that it is justbarely considered in accordance with the Equation 1 above in connectionwith the correction factor 2 log p Since carbon dioxide as Well ascarbon monoxide are measured as partial pressures, due to the dependenceof infra-red absorption on their density, and the absolute pressurewithin the furnace and thus Within the measuring chamber isapproximately constant over long periods of time, correction forpressure is not usually necessary.

In actual operation it has been shown that an error tolerance of i0.05%C can be readily achieved and frequently improved on. Automatic controlof maintenance of carbon potential can be obtained easily. The indicator15 can be formed with maximum and minimum contacts, activated by theindicator pointer. Alternatively,a continously effective comparator canalso be utilized. A manually settable potentiometer 52, is adjusted fora certain carbon potential value; it controls a potential which iscompared in comparator 51 with the ouput from amplifier 32., alsoapplied to indicator 15. Comparator 51 then controls a valvingarrangement, consisting of valves 53, 54 which, respectively, introducepropane or air into the atmosphere of the furnace. For example, if thecarbon potential value to too low, propane is added by opening valve 53;if the carbon potential is too high, a small quantity of air is added byopening of valve 54.

The determination as to whether to measure the partial pressure of watervapor, or carbon dioxide depends on the general ambient conditions ofeach application. Measuring the water vapor pressure has the advantagethat the measuring instrument 14 is comparatively inexpensive, becausean infrared analysis device need not be used. Measurements of watervapor pressure usually are based on measurements of steam pressure, sothat the result obtained is usually generally substantially proportionalto the Water vapor partial pressure (p Transducers to determine the dewpoint, or humdity, such as lithium chloride are comparatively simple andcheap, particularly in comparison with analysis apparatus for carbondioxide. One of the disadvantages of measuring the water vapor partialpressure is, that the vapor may condense within the ducting 13 (FIG. 1)when the dew point of the sampling gas exceeds the ambient temperature.It may thus be necessary to utilize heated ducts; arranging the filter12 in the sampling assembly head 11, in accordance with the presentinvention and as shown in FIG. 1 avoids the necessity of heating thefilter itself and avoids the major diificulties with condensation. Whenutilizing a lithium chloride element, the transformation temperature tof lithium chloride is measured directly, which is directly proportionalto the water vapor partial pressure p In order to compensate variationsof carbon monoxide and hydrogen components within the atmosphere of theoven, when utilizing the partial water vapor pressure as a base, a pairof potentiometers can be adjusted and utilized to introduce correctionfactors: one for variations of ambient atmospheric pressure, and theother for variations of the product of the volumetric portions of carbonmonoxide and hydrogen. Since the atmospheric pressure variations areusually slow, correction twice daily in accordance with barometricpressure is sufficient. The scale on the potentiometer can thus be setto indicate from 700 to 780 torr. Calibration is in accordance with theEquation 2 above.

When the humidity of the ambient air varies substantially, then furthercorrections can be entered by one or 10 additional potentiometer bridgenetworks as discussed previously in connection with FIG. 2. Such acorrection may be necessary if pure propane gas is used. The productvol. percent CO vol. percent H results in a value of 760 when purepropane and absolutely dry air is used; pure propane itself will yield avalue of 840. These values change by about 40 units for each percentageof humidity within the air in the furnace. Thus, compensation forambient humidity of air introduced may be necessary.

I claim: 1. Apparatus for determining the carbon potential of theatmosphere in a furnace, comprising:

means continuously determining the temperature of the atmosphere Withinthe furnace at a given point and deriving a corresponding electricaltemperature signal; means removing an atmosphere sample from immediatelyadjacent said given point at which said temperature is determined; meansanalyzing the concentration of the CO component of the atmosphere andcontinuously generating an electrical-gas-characteristic signalrepresentative of a defined function of the partial pressure of the COcomponent, said defined function being:

wherein:

c (in atomic fractions) is equal to concentration of carbon ingamma-iron; p, in atmospheres, is equal to partial pressure of the gascom-ponent in the atmosphere; T in degrees K. equals absolutetemperature in the furnace; and 7 is a correction coefiicient for aspecific alloy composition of a work piece in the furnace; meanscontinuously electrically adding said temperature signal and said gascharacteristic signal; means indicating a characteristic of saidcombined,

added signal to, continuously display the carbon potential directly;means intermittently generating an electrical error signalrepresentative of the error value between the carbon potential of astandard material sample, as derived from an intermittently analyzedsample and the carbon potential as indicated; and means adding saidelectrical error signal representative to said error value to said addedtemperature and gas characteristics signal to reduce any error value tozero.

2. Apparatus as claimed in claim 1 including signal generating meansconnected to said adding means to add further electrical signals to saidadded temperature and gas-characteristics signals, said furtherelectrical signals being representative of additional correction factorsobtained during operation of said furnace.

3. Apparatus according to claim 1, wherein said electrical error signalis generated intermittently in about weekly intervals and added in aboutweekly intervals to said added temperature and gas characteristicssignals.

4. Apparatus for determining the carbon potential of the atmosphere in afurnace comprising:

means continuously determining the temperature of the atmosphere Withinthe furnace at a given point and deriving a corresponding electricaltemperature sig nal;

means removing an atmosphere sample from immediately adjacent said givenpoint at which said temperature is determined;

means analyzing the concentration of the water vapor component of saidatmosphere and continuously generating an electrical gas-characteristicsignal representative of a defined function of the partial pressure 1 1of the water vapor component, said defined function being:

wherein:

c (in atomic fractions) is equal to concentration of carbon ingamma-iron; p, in atmospheres, is equal to partial pressure of the gascomponent in the atmosphere; T in degrees K. equals absolute temperaturein the furnace; and is a correction coeflicient for a specific alloycomposition of a Work piece in the furnace; means continuouslyelectrically adding said temperature signal and said gas characteristicsignal; means indicating a characteristic of said combined,

added signal to continuously display the carbon potential directly;

means intermittently generating an electrical error signalrepresentative of the error value between the carbon potential of astandard material sample as derived from an intermittently analyzedsample and the carbon potential as indicated; and

means adding said electrical error signal representative of said errorvalue to said added temperature and gas characteristics signal to reduceany error value to zero.

5. Apparatus as claimed in claim 4 including signal generating meansconnected to said adding means to add further electrical signals to saidadded temperature and gas-characteristics signals, said furtherelectrical signals being representative of additional correction factorsobtained during operation of said furnace.

6. Apparatus to determine the carbon potential in the atmosphere of afurnace comprising means to sense the condition of the atmosphere in thefurnace, said means being located within said furnace and including athermocouple (6, 7) to determine the temperature within a point in thefurnace and supplying a temperature signal;

a suction tube and means to suck gas from the furnace through the tube,said tube having a suction inlet located immediately adjacent theposition of the thermocouple within the furnace;

a gas analysis apparatus (14) connected to said suction tube to analyzegas sucked therefrom, said apparatus including means to sense thepartial pressure of the component of the gas sucked through the tube andproviding an electrical gascharacteristic signal;

a function generator having said gas characteristic signal and saidtemperature signal applied thereto and providing a transformedgas-characteristic output signal, for the gas being C wherein c (inatomic fractions) is equal to concentration of carbon in gamma-iron; vp, in atmospheres, is equal to partial pressure of the gas component inthe atmosphere; T in K. equals absolute temperature in the furnace; andis a correction coeflicient for a specific alloy compositon of a Workpiece in the furnace; manually settable electrical potential generatingmeans (29, 30, 31, 37) generating adjustment signals to represent'fixedfactors and to represent the error value between the carbon potential ofa standard material sample, as derived from an intermittently analyzedsample and the carbon potential as indicated; an adding circuitconnected to and electrically adding said transformed gas-characteristicsignahsaid' tem- 12 perature signal and at least one of said adjustmentsignals and providing an indicating signal; and an indicating means (15)connected to said adding circuit and having said indicated signalapplied thereto and being directly readable in percent of carbonpotential.

7. Apparatus according to claim 6 wherein said sensin means includes agas filter (12) located immediately adjacent the inlet and of saidsuction tube.

8. Apparatus according to claim 6, wherein said sensing means includes asensing assembly introduced into said furnace; and said sensing assemblyincludes a test sample tube to hold a standard sample, said tube havingperforations to establish communication between the interior of the testsample tube and said furnace to expose said test sample to theatmosphere inside the furnace.

9. The apparatus as claimed in claim 6, wherein said manually settablepotential generating means includes sources of potential and manuallysettable Potentiometers (16, 17, 18, 38).

10. The apparatus as claimed in claim 6, wherein said manually settablepotential generating means includes means generating a fixed potential(33, 34, 35, 37); seriesconnected bridge circuits (29, 30, 31, 37)having one diagonal connection connected to said sources of fixedpotential and the other diagonal connections connected in series; andmanually settable potentiometers (16, 17, 18, 38) in a branch of each ofsaid bridge circuits.

11. The apparatus as claimed in claim 6, wherein said means to sense thepartial CO pressure includes an infrared spectrum-absorption apparatus.

12. The apparatus as claimed in claim 6, including control meansconnected to said indicating means, valve means regulating thecomposition of the atmosphere within said furnace; said control meanscontrolling the valve means to keep the carbon potential Within thefurnace at a desired point by addition of air or gas capable of changingthe carbon content of the atmosphere within the furnace.

' 13. Apparatus to determine the carbon potential in the atmosphere of afurnace comprising means to sense the condition of the atmosphere in thefurnace, said means being located within said furnace and including athermocouple (6, 7) to determine the temperature within a point in thefurnace and supplying a temperature signal;

a suction tube and means to suck gas from the furnace through the tube,said tube having a suction inlet located immediately adjacent theposition of the thermocouple within the furnace;

a gas analysis apparatus (14) connected to said suction tube to analyzegas sucked therefrom, said apparatus including means to sense thepartial pressure of the component of the gas sucked through the tube andproviding an electrical gas-characteristic signal;

a function generator having said gas characteristic signal and saidtemperature signal applied thereto and providing a transformedgas-characteristic output signal, for the gas being H O component:

wherein c (in atomic fractions) is equal to concentration of carbon ingamma-iron; p, in atmospheres, is equal to partial pressure of the gascomponent in the atmosphere; T in K. equals absolute temperature in thefurnace; and 'y is a correction coefficient for a specific alloycomposition of a work piece in the furnace; manually settable electricalpotential generating means (29,-30, 31, 37) generating adjustmentsignals to represent fixed factors and to represent the error valuebetween the carbon potential of a standard material sample, as derivedfronr-and intermittent- 13 ly analyzed sample and the carbon potentialas indicated; an adding circuit connected to and electrically addingsaid transformed gas-characteristic signal, said temperature signal andat least one of said adjustment 5 signals and providing an indicatingsignal; and

an indicating means (15) connected to said adding circuit and havingsaid indicated signal applied thereto and being directly readable inpercent of carbon potential.

14. The apparatus as claimed in claim 13, wherein said means to sensethe partial H O pressure is a lithium chloride humidity indicator andincludes a bridge network (25) having one branch thereof formed by ahumidity sensitive resistance (14a); said adding circuit includingconnection means interconnecting one diagonal connection of said bridgecircuit with said thermocouple.

References Cited UNITED STATES PATENTS 2,935,866 5/1960 Schmidt 73272,949,765 8/ 1960 Thayer 7327 2,648,976 8/1953 Bur 73--23 3,128,3234/1964 Davis 148-165X 3,070,990 1/1963 Krinov 73-25 3,237,928 3/1966Warman 14816.5X 10 3,329,495 7/1967 Ohta 73-23X OTHER REFERENCES SteelCarburization and Decarburization-J. K. Stanley, The Iron Age, J an. 21,1943, pp. 31-35.

15 RICHARD c. QUEISSER, Primary Examiner C. E. SNEE III, AssistantExaminer

